Arc Breakdown Research Articles

Submerged Arc Breakdown of Sulfadimethoxine (SDM) in Aqueous Solutions

Low voltage, low energy submerged pulsed arcs were used to break-down Sulfadimethoxine (SDM) contamination in aqueous solutions. The SDM concentration decreased exponentially with rate constants of 0.13-1.9 min -1 during processing by pulsed arcs with a pulse repetition rate of 100 Hz, energies of 2.6-192 mJ and durations of 20, 50 and 100 ls. The electrical energy consumption was minimized with short duration pulses--1.5 kW-hr/m 3 with 7.5 mJ, 20 ls pulses for 90% SDM removal.

Green-laser-triggered Water Switching at 1.6 MV

Multiple water switches are used in the self-breaking mode in many large pulsed power systems. We are studying laser-triggering of water switches at voltages of up to 1.6 MV to see whether we can lower the command jitter of water switches. We have previously reported studies of 170 kV water switching with command jitters as low as plusmn2 ns. The 1.6 MV triggering experiments reported here are performed on a water switch in the middle of a 1.8 m long 7.8 Omega coaxial water line that is directly charged by a 65 kJ Marx generator. The 10 to 90% rise-time of the sinusoidal pulse impressed across the water switch is 250 ns. To trigger the switch, we transport a green laser beam (0.4 J, 7 ns pulsewidth) radially inward though the water of the coaxial line to a 'dry box' inside the inner coax line. There, the laser beam is turned 90 degrees and focused through a hole in one electrode to a breakdown arc in the water between the switch electrodes. Best results, of plusmn8.3 ns jitter and 100 ns delay at 60% of the self-break voltage, have been achieved using an axicon lens to focus the beam to a long narrow chain of point breakdowns between the switch electrodes.

Arc erosion behavior of a nanocomposite W-Cu electrical contact material

Abstract The erosion behavior of a nanocomposite W-Cu material under arc breakdown was investigated. The arc erosion rates of the material were determined, and the eroded surfaces and arc erosion mechanisms were studied by scanning electron microscopy. It is concluded that the nanocomposite W-Cu electrical contact material shows a characteristic of spreading arcs. The arc breakdown of a commercially used W-Cu alloy was limited in a few areas, and its average arc erosion rate is twice as large as that of the former. Furthermore, it is also proved that the arc extinction ability and arc stability of the nanocomposite W-Cu material are excellent, and melting is the major failure modality in the make-and-break operation of arcs.

Effects of Postpulse Surface Temperature on Micropulsed Plasma Thruster Operation

High-speed mercury cadmium telluride photovoltaic detectors sensitive to infrared emission are used in a micropulsed plasma thruster (micro-PPT) to explore the surface-temperature profile throughout the discharge process for an ablative arc over Teflon. Real-time surface-temperature measurements are made after the current pulse ends in a micro-PPT to examine heating methods and ablation characteristics of the Teflon® propellant. Calibration is performed on heated Teflon, experimentally accounting for temperature-dependent variations in emissivity. Time-dependent measurements are taken of the fuel face and used to determine a surface-temperature profile after the arc. Photographs of the arc breakdown are taken with a gated, intensified camera capable of 5-ns shutter times allowing visual interpretation of the arc emission in terms of nonaxisymmetric arc spoking effects. The data are analyzed to calculate Teflon vapor pressure and expected impulse bit contribution of the neutral vapor liberated from the fuel face during the ablative cooling process. The cooling profile is also compared with predictions from a code that is under development. Finally, methods for using this sensing technique during the current pulse of the thruster are proposed and evaluated for future applications.

Sound emission from an arc cathode

Sound waves have been detected from an electrode acting as the cathode in a vacuum discharge during electrical breakdown and subsequent arcing. Results from arcing on graphite, carbon fiber reinforced carbon, and stainless steel are reported. Distinct frequencies could be reproducibly detected in the power spectrum belonging to different groups. Most of the modes show a strong damping demonstrating that the nonlinear disturbance associated with the breakdown excites a wide spectrum. To identify eigenoscillations of the cathode plate some experiments have been repeated using an electrode with modified geometry. Different disturbances during the lifetime of the moving arc are reflected in different selections of sound eigenmodes.

Polarizers with non-rectangular grooves for high power millimeter waves

Polarizers with non-rectangular grooves are studied in high power millimeter wave transmission lines for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) of fusion plasmas. The groove shape is important for determining the polarization parameters and avoiding arc breakdown in the system. A low-power measurement has been carried out for several polarizers with different groove depths. The polarization characteristics experimentally measured are in good agreement with numerical results in which the actual groove shape is taken into account. The polarizers are designed and applied to different frequencies of ECH/ECCD systems. Favorable results have been obtained in high-power transmission up to 500 kW, 0.2 s.

Further development of a gaseous ion source based on low-pressure hollow cathode glow

The gaseous ion source based on a hollow cathode glow discharge with additional external injection of electron beam described in a previous publication has undergone further development. The direction of the source upgrade was to increase the total beam current and its density keeping the same broad beam cross section (about 100 cm2). With an operating gas pressure of 10−4 Torr, the maximum stable discharge current was as high as 40 A in 300 μs (pulsed mode) and about 10 A (dc mode) without discharge gap arc breakdown. The total ion emission current exceeded 1 A in both cases. The geometry of the discharge gap was optimized, allowing improvement of the parameters of the device. The composition of the ion beam under various operating conditions of the discharge has been measured using the time-of-flight method. The electron beam injection into the hollow cathode of the ion source resulted in a reduction of the discharge voltage from the usual 500–600 V to 100 V or less. This lead to lower sputtering and as a result low contamination of the gaseous ion beam by metal fractions. Measurements showed 0.15% metal content instead of the typical several percent. The influence of the ion beam extraction on the discharge parameters was also examined.

Thermionic arc breakdown in small discharge gaps: model and applications

A model for thermionically induced breakdown is developed which predicts the voltage levels experienced in low-voltage discharges. Initially, electron emission across an active discharge minigap must be established. Prior to breakdown, emitted electrons populate the cathode space-charge region and the remaining gap is unionized. Breakdown criteria and a means for estimating the reduced breakdown voltage in the bulk gas are derived in terms of conventional Townsend ionization mechanisms, wherein multistep or cumulative ionization must be active. Since the cathode space-charge region is replete with emitted electrons, positive ions traversing it release to the bulk gas considerably more electrons than would be available with the conventional Townsend /spl gamma/ mechanism at the cathode surface. This equivalently higher /spl gamma/ significantly shifts the Paschen curve toward lower minima. One practical thermionic assisting device is a tungsten light-bulb-grade filament, which shunts the primary discharge electrodes and, within the filament's minigaps, breakdown occurs. Thermionic arc breakdown has been implemented in the starting of arc devices for lamps and in thermionic power converters; an application to ionization chambers in electrostatic thrusters is also discussed.

Electrical breakdown of small gaps in vacuum

The comparison of the characteristics of an electric arc breakdown in vacuum between two silver or silver-nickel alloy convex electrodes, enabled us to show experimentally that the process initiating discharge in vacuum is the electronic emission at the micrometer scale. Increased content of nickel in the alloy improves the insulation and achieves high resistance to erosion for low power arcs. Heating in vacuum demonstrates the multiple layer structure of studied alloys. Secondary electron microscopy and energy dispersing spectroscopy of the electrode surfaces after breakdown, as well as the measurements of fall and delay times, enabled us to evidence the origin of the electric arc.

Effect of target temperature on the reactive d.c.-sputtering of silicon and niobium oxides

We study the deposition of Si and Nb oxides by reactive d.c.-magnetron sputtering from high temperature metallic Si and Nb targets in Ar/O 2 atmospheres. It is shown that the oxidess formed on Si targets above 1 500 K and on Nb above 2 310 K evaporate, leaving the target surface free of the solid oxide coatings which induce hysteresis, voltage instability and arc breakdown in conventional reactive sputtering. Also, the deposition rate can be much higher because the target surface is coating-free and because sublimed oxides, SiO(g) for Si targets and a mixture of NbO(g) and NbO 2(g) for Nb, contribute significantly to the deposit: The deposition rate of SiO 2 is three times higher than for a conventional process with the same power and for Nb 2O 5, it is seven times higher. Thermionic emission is shown to be an important process which can constitute more than half of the total discharge current in the hot target sputtering of Nb.

Control of discharge initiation in high-intensity electron-beam generators for metal processing

Electric fields on the different components that are involved in the high-voltage performance of an electron gun used for welding are evaluated by a numerical method in order to reduce the risk of discharge breakdown during normal operation. A system composed of a tetrode tube placed in series between the power source and the electron gun and acting as a ballast resistor is developed to detect the initiation of a discharge in the electron gun and to stop its evolution toward a breakdown arc. The effect of beam interruption time on metal solidification during welding is addressed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The epitaxial growth of microprotrusions on field emitter tips

One precursor to cathode initiated vacuum arc breakdown is the growth of microprotrusions on the surface of the cathode. Ion bombardment activated surface atom transport in the presence of a large applied electric field has been associated with microprotrusion growth. Previous low-temperature cathode bombardment investigations have been extended into a cathode temperature regime of ∼0.2Tm–0.4Tm, where Tm is the cathode melting point. At cathode temperatures of ∼0.3Tm, helium ion cathode bombardment initiates microprotrusion growth which is epitaxial and confined to particular crystal planes. This form of epitaxial growth is partially associated with the thermally activated component of surface transport present at ∼0.3Tm.

Laser triggering of a 500-kV gas-filled switch: A parametric study

We have investigated neodymium: yttrium aluminum garnet (Nd:YAG) laser triggering of a 500-kV, SF6-insulated gas switch for a range of laser parameters. Laser wavelengths of 266 nm and 1064 nm with nominal pulse lengths of 2- and 4-nsec full width at half maximum (FWHM) were used to trigger the switch. The switch was triggered by focusing the laser to form a breakdown arc in the gas between the electrodes. Subnanosecond jitter in the operation of the switch was obtained for 266-nm laser pulse energies as low as 5 mJ. Results obtained with the Nd:YAG laser are compared to earlier data in which the switch was triggered with a krypton-fluoride laser operating at 248 nm with a pulse length of 20-nsec FWHM. Our experimental results show that ultraviolet laser triggering gives results which are a dramatic improvement over infrared laser triggering in terms of lower jitter and smaller change in delay time with variations in applied voltage. We also see indications that the optimum laser pulse length is equal to or greater than the closure time of the switch.

A Dynamic Model of Thundercloud Electric Fields

A dynamic interactive computer model of the electrical behavior of a thundercloud surrounded by the distributed atmosphere, earth, ionosphere circuit is described. The electrification mechanisms in the model are represented by current or voltage generators. The electrical breakdown is taken care of by switches that close when the voltages between nodes correspond to assumed breakdown electric fields and open again when the voltage fall below those necessary to sustain the arc. The model is thus able to simulate the very complicated interactive processes that occur in and around a thundercloud due to the linear conduction and displacement currents, as well as the nonlinear charge redistribution processes that occur during arc breakdown within and outside the cloud. The new models are intended to serve as an extension of the convective dynamic and microphysical models. They allow predictions to be made of the interrelation of the observable electrical parameters of the storm to the geometry and parameters of the electrical generating mechanisms. It is shown that there are major differences between the behavior of a fully interactive dynamic model and the simple monopole or dipole models on the one hand and models which neglect atmospheric conduction and displacement currents and electrical breakdown on the other. To demonstrate the possibilities of this type of model, several exploratory studies were carried out. The effects of variations in the height of the cloud above ground and of the vertical separation between the top and bottom of the cloud were investigated. Changing the cloud conductivity from one tenth to ten times the conductivity of the surrounding atmosphere was shown to have only a small effect on the external electric fields and on the current to the ionosphere. Several generator configurations were examined to study the effect of partial neutralization of the precipitation at the bottom of the cloud. The effect of neutralization between the cloud and ground was also examined. It was shown that only in cases when the cloud current was almost completely neutralized in the bottom of the cloud or when almost all the current flowed to the ground could cloud-to-ground strokes occur.

Flashover breakdown of an insulator in vacuum by a voltage impulse in the presence of a magnetic field

The flashover strength of Plexiglas insulators subjected to submicrosecond voltage impulses in vacuum and in the presence of a magnetic field that is perpendicular to the insulator surface is studied. Flashover breakdown strength is reduced by magnetic fields of a few kilogauss. Observation by fast-framing photography does not reveal curvature of the breakdown arc greater than 20°.

Continuous uniform excitation of medium-pressure CO2 laser plasmas by means of controlled avalanche ionization

A sequence of 50- to 75-nsec high-voltage pulses in combination with photoionization produce spatially uniform continuous electron densities of 2 × 1011 with a 10–20% ripple at 20 000 Hz in 150-Torr CO2 laser mixes. A separate continuous electric field ranging from 3 to 6 kV/cm atm is used to heat the electrons, which loads greater than 0.25 J/cm3 atm into the plasma without arc breakdown. As is the case with electron beam plasmas, E/P and ne may be controlled independently. Hence, electron beams, flow constrictions, etc., are not necessary to produce uniform plasmas for cw high-power electric gas laser systems.

12.2 - Laser-triggered megavolt switching

Laser-initiated breakdown of high-voltage spark gaps has been extended to the megavolt range through the use of a novel coaxial triggering geometry. In this configuration, a 100-to 250-megawatt ruby laser of 1.4- to 3.5-joules output was aligned along the interelectrode axis of the spark gap. The laser beam passed through a hemispherical electrode, mounted on a hollow shaft, and was focused by a lens internal to this electrode at the opposite-switch electrode surface. Based on investigations of the effect of polarity on switch performance, i.e., least delay and jitter, it was concluded that 1) irradiation of the charged electrode was preferred and 2) irradiation of positive-rather than negative-charged electrodes gave best performance. Delay times between arrival of the laser pulse and complete gap closure as short as 2 ns with unmeasurable jitter (< 1 ns) were readily attainable under various conditions (high-pressure and high-reduced fields). By recourse to classical arc breakdown theories, i.e., Townsend avalanche and streamer mechanisms, it was concluded that the variation of delay with reduced field follows an avalanche process. However, for an explanation of extremely short delays observed (high-velocity closure rates), a streamer mechanism is necessary. Photographs of laser-triggered breakdown gave consistently single-channel straight arc breakdown with the absence of lossy arrested discharges, while self-breakdown or mechanically triggered breakdown produced longer and irregular breakdown channels with numerous energy-absorbing arrested leaders. This observation is in concert with the extremely short delays observed and leads us to suspect that volume dielectric breakdown or at least arc channel preparation is responsible for the observed results.

Streamer Mechanism in Filamentary Spark Breakdown in Argon by Fast Photomultiplier Techniques

Using techniques developed by Hudson on air and reported in the preceding paper, the phenomena were studied in Linde's spectroscopically pure grade Ar gas admitted to a system using Alpert vacuum techniques, and on that gas further purified by gas cataphoresis. Study was made in a point-to-plane gap with a 2.36-mm hemispherically capped cylinder opposite a 3-cm distant thin out-gassed Ni plane in the pressure range from 300 mm to 50 mm. For the spectroscopically pure grade Ar, transition from a positive point corona through a fine filamentary spark to an incipient arc breakdown on a time scale of ${10}^{\ensuremath{-}7}$ sec down to 100 mm pressure proceeds by movement of primary and secondary streamers progressing from anode to cathode at slower speeds and lower luminous intensities than air at 760 mm. Unlike the case for air, the main stroke in Ar appears to move from anode towards the cathode. At 50 mm a somewhat diffuse spark channel did not reveal any streamer-like progression but the time scale was still in the ${10}^{\ensuremath{-}7}$-sec range. One percent air in Ar at 60 mm restored streamers. Purified Ar at 240 mm revealed a 2-mm wide diffuse channel breakdown occurring across the whole gap by a process unknown with a rise time of several microseconds and sustained luminosity for tens of \ensuremath{\mu}sec with no indication of streamers. This demonstrates the necessity of adequate photoionizable impurities in Ar for the development of the filamentary streamer spark transition.

Contribution to the Problem of Arcback in High Voltage Discharge Devices

It has been found in rectifier and ignitron practice that arcback can occur after deionization of the discharge path at such vapor pressures where no self-sustaining glow can develop. Experiments to study this kind of an arc breakdown were made in Hg vapor vessels between cold electrodes at voltages between 10 and 60 kv. It appeared that the start of arc breakdown was not dependent on the Paschen law. At the voltages used and distances between the electrodes of less than 50 cm arc breakdown always started at PD values which were much lower than those required to sustain a glow discharge. This is contrary to earlier experience at lower voltages. With rising voltage arc breakdown frequency increased in an exponential manner. Changes in vapor pressure and electrode spacing had no definite effect on arc breakdown frequency as long as glow discharge conditions were not approached. Changes in partial air pressure did not affect arc breakdown frequency if air pressure was below 10 percent of mercury vapor pressure. Heat treatment of the cathode (graphite ``anode'') has a large effect in reducing arc breakdown. Even well heat-treated ``anodes'' show seasoning effect, i.e. steady decrease of arc breakdown frequency after initial maximum. Phenomena on cathode surface are believed to be responsible for formation of arc breakdown.